The invention relates to regenerative heat exchangers and more particularly to the regenerative mass of such heat exchangers.
Regenerative heat exchangers of the rotary type are commonly used as air preheaters in boiler plants and in most cases the regenerative mass of the preheater consists of profiled plates carried by an annular regenerator body and arranged such as to form channels for the heat exchanging fluids which channels extend between the axially opposite ends of the body. There are two main types of rotary air preheaters, one having a rotatable regenerative mass and stationary ducts for the heat exchanging fluids and the other having a stationary regenerative mass and rotatable ducts for the fluids.
Already at an early stage of the development of the rotary regenerative heat exchanger it was found that it was necessary to use profiled plates in order to increase the heat transfer between the fluids and the plates to an acceptable level. Thus, during the past decennia there have been proposed several plate shapes. In modern regenerative heat exchangers the regenerative mass is most frequently composed of plates which are provided with undulations or corrugations extending obliquely relatively to the main direction of fluid flows through the mass and the plates are further provided with ridges which are parallel with said main flow direction and serve as spacers between adjacent plates. Such undulated and ridged plates alternate with plane or undulated plates to form channels for the fluid flows to which is imparted a certain turbulence by the undulations.
In air preheaters of the regenerative type air and flue gases pass in countercurrent relationship through the regenerative mass. Thereby the temperature of the mass becomes much higher at the gas inlet and air outlet end than at the gas outlet and air inlet end. During operation it is of course desirable to extract as much heat as possible from the gases. However, if the temperature of the gases reaches the dew point corrosive moisture is deposited on the surfaces of the plates resulting in rapid destruction of the sheet metal if it is not corrosion resisting in itself or protected against corrosion.
It has proved that corrosion may occur here and there at the cold end of the regenerative mass even though the temperature of the gases in the outlet duct has never dropped to the dew point. It has been found that this local corrosion is caused by an uneven distribution of the fluid flows in the separate channels or groups of channels. If, for instance, due to the oblique undulations the gas flow gets a tendency to concentrate at one side of the channel the air flow gets a corresponding tendency to concentrate at the other side of the channel. This results in an uneven temperature distribution in the gas flow leaving the channel so that the temperature at one side is above and at the other side may lie at or below the dew point but nevertheless the average temperature of the flow may lie well above the dew point.
One object of the invention is to provide a pack of heat transfer plates for regenerative heat exchangers which is composed of identically profiled plates.
Another object of the invention is to provide a plate pack in which the temperature of the plate portions defining a channel is substantially constant along the perimeter of the cross section of the channel at least at the cold end of the regenerative mass.